Diagnose engine compression issues like a pro with this Redline Update - Hagerty Media
The Hagerty employee-restored 1969 Chevrolet Camaro is not running well. It has racked up miles driving from sea to shining sea, but the latest trip has brought it to the Redline Rebuild garage for Davin to diagnose excessive oil consumption and poor running. That calls for a compression test, but Davin takes it one step further and completes a leakdown test as well—showing you exactly how to do it yourself along the way.
This particular 396-cubic-inch big-block Camaro has been on the road for five years since its restoration. In that time it’s done everything from short test drives to the 2,100-mile Great Race, and is starting to act a bit tired. On top of that, the big block is consuming 10W-30 like Joey Chestnut eats hot dogs.
Davin starts the diagnosis by pulling the spark plugs, as their condition can give a lot of information about the overall health of an engine. These plugs? Not telling a good story.
“The build-up of carbon and oil on the electrodes and insulators of a few of them was real bad,” Davin says. “I could imagine there was at least one cylinder that was not firing, possibly two. With that much oil on them, I had to dig deeper to see exactly where it was coming from.”
That leads to a compression check, which can tell if a cylinder’s pressure is bled off past the piston rings, valves, or into the water jacket or oil passages through the head gasket instead of sealing tight. If the compression is down, like what Davin experiences on cylinder number three, a small amount of oil can be added to the cylinder before completing the compression check again. This oil in the cylinder will help seal the rings during the short test and confirm or deny the hypothesis of tired piston rings. If the compression goes up during the wet test, it confirms the rings need to be replaced.
Even with the compression check telling him enough information to require an engine-out rebuild, Davin takes the extra step and gives the big-block a leakdown test as well. Using a different fitting in place of the spark plug, Davin uses compressed air and a small fixture to measure the amount of compression drop. With sustained pressure in the cylinder, one can usually hear any air leaks, which typically come out the exhaust, intake manifold, or crankcase ventilation.
The short answer is this big-block is headed for the engine stand and a full once-over before it gets back out into the streets. Will there be even more damage hiding inside the engine? With the test results showing it has been run low on oil and possibly overheated, the chances are high. To find out for sure, though, you’ll have to tune in to the next episode of Redline Update.